1. Field of the Invention
The present invention relates generally to a shift valve arrangement for use in an automatic transmission and more specifically to a shift valve arrangement which permits the transmission to downshift directly between the first and last of three sequential speeds when the transmission is subject to coasting and which provides an adequate separation between the first-second and second-third upshifts shifts under low load.
2. Description of the Prior Art
JP-A-58-156757 discloses a four (forward) speed transmission wherein a one-way clutch is utilized during first speed. With this arrangement when the transmission undergoes a 1-2 upshift or a 2-1downshift, the one-way clutch changes from an engaged state to a released one and vice versa, and provides for relatively smooth shifting. When the vehicle is coasting and the transmission downshifts, rather than a sequential 3-2-1 downshift, a direct 3-1 shift tends to produce a smaller shift shock. The reason for this is that a 3-2 downshift does not involve the one-way clutch and the amount of shift shock tends to be relatively large. On the other hand, a 3-1 downshift induces the engagement of the one-way clutch and as a result a smaller amount of shock tends to be produced.
However, with this type of transmission even though the transmission produces relatively little shift shock, if induced to produce a 3-1 downshift under vehicle coasting, in order to enable such a shift, the 3-2 shift line has to be set lower than the 2-1 shift line in the low throttle opening region. However, if the 3-2 downshift line is set in this manner, the 2-3 upshift line is drawn into close proximity thereto in the low thottle setting region. In some instances this actually establishes a 1-3 upshift line. Alternatively, if the 1-3 upshift line is not established, the 2-3 upshift occurs immediately after the 1-2 upshift. In either case the shift feeling is deteriorated.